The present invention relates to methods, apparatuses and systems for determining one or more physical dimensions of an entity, more particularly for determining the thickness of a thin material overlayer on a solid surface.
There are a number of approaches for determining the thickness of a thin carbon overlayer on a sample surface. Several approaches involve the use of x-ray photoelectron spectroscopy ("XPS").
An example of carbonaceous overlayer thickness determination implementing XPS involves ion beam depth profiling. In accordance therewith, the sample surface is gradually eroded away using an ion beam, and subsequently analyzed with XPS, or Auger electron spectroscopy ("AES"). Performing this process iteratively yields a depth profile of the surface.
Angle-resolved XPS represents another example of XPS-based carbonaceous overlayer thickness determination. According to angle-resolved XPS, photoelectrons are detected at multiple angles with respect to the sample surface. This results in different photoelectron path lengths through the carbon overlayer. From this data, the layer thickness can be determined.
Another XPS-based approach for determining the thickness of a thin carbon overlayer on a sample surface uses two different photoelectron lines with widely differing energies from the same element, in either the substrate or the overlayer. The relative attenuation of the two lines can be used to determine the overlayer thickness.
There are also non-XPS layer thickness techniques, such as ellipsometry.
XPS is a technique by which the elemental composition and chemistry of a solid surface is determined. The surface is illuminated by soft x-rays, resulting in the emission of photoelectrons. XPS is sensitive only to the top few atomic layers of a surface because the photoelectrons have mean free paths of only a few nanometers. XPS can be used to identify the elemental composition of a surface because the photoelectrons have kinetic energies characteristic of the elements in the sample. In addition, XPS can be used to characterize the chemical state of these elements because different chemical states give rise to measurable photoelectron kinetic energy shifts.
The determination of the thickness of thin carbonaceous overlayers on solid surfaces has important technological applications and implications.
For instance, in order to properly determine, using XPS, the composition of a substrate, the presence of an adventitious carbon layer (overlayer) must be accounted for. All samples possess an adventitious carbon layer a few nanometers thick on their surface; however, the adventitious carbon layer on a sample surface can drastically change the quantitation obtained with XPS, because the adventitious carbon layer absorbs photoelectrons from different elements (differing kinetic energies) with different efficiencies. This effect can be corrected for, but only if the thickness of the adventitious carbon layer is known. Since the overlayer thickness is a necessary aspect of this type of substrate composition determination, a speedy and practical method of determining overlayer thickness would be quite beneficial.
Moreover, biofouling of surfaces immersed in seawater is an extremely costly problem for the U.S. Navy. The prevention of such films is of great concern to the U.S. Navy. An effective technique for determining carbonaceous overlayer thickness may help characterize the initial stages of biofilm formation.
Also, there exist certain samples that possess an intentionally formed carbonaceous overlayer on their surface. For example, magnetic disk drives possess a thin lubricating layer on their surface. There is a need in the disk drive industry to measure the thickness of this layer.
Furthermore, layers of small organic molecules on a surface are used as crosslinkers to attach a biological molecule to a surface, as in a biosensor. Layer thickness determination could provide a valuable strategy for characterizing this crosslinking layer.